Junos OS

First of all please forgive me if I cause confusion on this and let me know if I can clarify things more..

I come from a Cisco world and am learning JUNOS. I have a question in regards to CoS markings on packets. In Cisco devices I can modify Layer2 or Layer3 CoS header information INGRESS an interface. From my reading in Juniper Devices you can only write that information EGRESS an interface and it comes from the 'rewrite-map'.

With Juniper devices you apply an input firewall filter that matches the traffic and then you define it to a forwarding class. Traffic is then forwarded through the device and once it reaches its egress interface using the rewrite-map it marks the packet CoS information based on the forwarding-class the packet was defined to. Also as we know, if filters aren't applied to force traffic forwarding classification the 'classifier' map is used to correlate the CoS markings to forwarding classes by default. We also know that if a rewrite-map isn't defined the traffic passes out and interface unmodified.

Here's my question. Say I have a router with 3 interfaces, 2 interfaces are input and 1 output. Interface #1 and #2 are input and #3 would be output. On interface #1 I want to mark the traffic as its currently unmarked and I want it marked to DSCP EF(46). I have to apply the firewall filter and define this traffic into the expedited forwarding class. To make traffic egress of the router have this marking I have to also apply the dscp rewrite-map on interface #3. On interface #2 the traffic is already marked to DSCP43. As I do not have a firewall filter applied, the default classifer map kicks in and maps the DSCP 43 traffic to expedited forwarding class as well. Once this traffic exits the router out of interface #3, the rewrite map that had to be defined for interface #1 will rewrite this traffic to DSCP 46, overwriting my original markets. Now I cannot differentiate the traffic further on in the network.

I see this is as a big limitation. Are there workarounds that I'm missing?

Hopefully someone can help..

Thanks!

Chris

I think you understand very well. You are correct that rewrite is per queue. That means that on egress per queue we cannot differentiate traffic that requires rewrite and traffic that does not. One way around this is to specify another queue specifically for the traffic that requires rewrite and one for traffic that does not require rewrite. On most JUNOS platforms you can specify 8 queues. Many CoS environments use 4 queues (BE, AF, EF, NC). That leaves room for custom queues that you can create.  Since queues are only important on each individual router, there should not be a reason why you cannot create more queues. 

Hope this helps.

-Richard